We wish to understand regulation and control mechanisms in immune systems and how microorganisms influence or deregulate these systems. In order to do this we have constructed monoclonal lines of immunocytes (hybridomas) which express the functions of regulatory T-cells. Cytotoxic T-cell clones were derived by selective growth in the presence of T-cell growth factors (TCGF) and IL2 with the continuous presence of stimulating antigen(s). We have constructed hybridomas from splenocytes of nude mice bearing T-cell markers. Some of these express surface markers and functional phenotypes which allow us to classify them as model precursors of regulatory T-cells. Modulatory effects of bacterial toxins such as the streptoccai exotoxin, SPE and pertussis toxin (PT) have been tested in parallel on these cell lines in vitro. Our previous experiments suggested that SPE deregulates the down regulation of T-cell dependent antibody responses. Current experiments using the monoclonal T-cell lines show that the effective target of SPE responsible for the observed activity is a precursor of T-cells rather than mature suppressor T-cells. Both SPE and PT are suppressive for CTL responses. Since CTL clones are not suppressed, it is suggested that precursors or regulators of CTL may be the target of SPE in the effector T-cell immune system as well. Pertussis toxin is supressive for both development of B-cell responses (PFC) and CTL. The target cells of PT and SPE appear to be different since PT is inactive on subpopulations of splenocytes containing precursors and active suppressors. At the molecular level, the genomes of the precursor-type T-cells from nude mice, appear not to be rearranged from germ-line or they are rearranged differently as compared with T-cells clones from wild type. The variable expression of T-cell surface markers by T-cell hybridomas has been associated with the coordinate expression of both allelic markers of the fusion parent cells (NFR x BW5147/AKR).